A transducer may be broadly defined as a device which converts one form of input energy to a different form of output energy. An electromechanical transducer, when configured to convert electrical energy to mechanical energy, may operate on the principle of electrostatic attraction caused by two opposing and oppositely charged conducting plates. For example, when electrical energy is input to the transducer in the form of a voltage applied between the plates, the plates are drawn together. If the plates are free to move together, the input electrical energy is converted into mechanical energy.
The plates of an electromechanical transducer may also be used to generate electrical energy from an input of mechanical energy. For example, the plates are first charged by an electrical voltage applied to the plates. The plates may then be disconnected from the charging source and mechanical energy used to further separate the plates. As the plates are separated, the voltage between the plates increases thereby converting the mechanical energy to electrical energy.
Accordingly, an electromechanical transducer may be used either as an actuator, sensor or power supply. As an actuator, the transducer may convert electrical power into mechanical motion, and, as a sensor or power supply, the transducer may convert mechanical motion into an electrical signal.
Electromechanical transducers have been developed which convert electrical energy to mechanical motion and ultimately to acoustic energy by the application of a voltage between a pair of spaced parallel conducting plates. If the plates are flexible or otherwise configured to allow motion, the plates are drawn together by the force of electrostatic attraction when the signal voltage is applied between the plates. See, for example, U.S. Pat. No. 3,008,014, to Williamson et al., which discloses an electrostatic transducer used in entertainment loudspeaker systems to convert electrical signals into sound. Since the driving voltage required to move the plates is related to the square of the separation between the plates, transducers of the type described in Williamson et al. require large and potentially hazardous driving voltages.
As is known in the art, the force generated by a pair of opposing parallel charged plates is inversely proportional to the square of the distance between the plates. The force generated by the plates increases by a power of two for a corresponding linear decrease in the separation between the plates. Accordingly, very large forces can be developed as the spacing between plates is decreased. In addition, for a given force, as the separation is decreased, the driving voltage can be reduced. Accordingly, there has been widespread interest in the development and manufacture of microminiature electromechanical transducers, or "microelectromechanical" transducers.
To obtain useful forces and physical displacements as the size of the separation between plates is reduced, a large number of plates must be concatenated or stacked together. An early attempt at fabricating a microelectromechanical transducer is described in U.S. Pat. No. 2,975,307, to Schroeder et al. This patent discloses an electrostatic transducer having a large number of stacked plates, each plate with an individual and discrete external wiring connection to the source of the driving voltage, and each pair of adjacent plates having a series of separators positioned in a precise pattern therebetween.
Unfortunately, it is difficult to connect each of the plates to a supply voltage in an array having a large number of closely spaced plates. In an array of closely spaced stacked plates, many hundreds or even thousands of discrete connections must be made to each plate in the stacked array of plates. In addition, the physical assembly of such a large number of plates, spacers, and other components of such small dimensions is extremely difficult and not, therefore, amenable to efficient manufacturing.
The microelectronics art has been highly successful in fabricating extremely dense microelectronic structures. For example, one million or more transistors have been fabricated on an integrated circuit chip less than 1 cm.sup.2 in area. Accordingly, attempts have been made to use microelectronics manufacturing principles and techniques to fabricate microelectromechanical transducers.
A major advance in the design and manufacture of microelectromechanical transducers is described in U.S. Pat. No. 5,206,557, entitled Microelectromechanical Transducer and Fabrication Method to coinventor Stephen M. Bobbio, the disclosure of which is incorporated herein by reference. An electromechanical transducer having a large number of conductive strips with a small separation between adjacent strips is described. This transducer avoids the need for individual discrete wiring to each strip.
The transducer of Pat. No. 5,206,557, is formed of a plurality of electrically conductive strips arranged in an array, with adjacent portions of the strips being maintained in a closely spaced relation by a series of spacers positioned between the adjacent portions of the strips. The spacers have electrically conductive portions to distribute the electrical signal within the transducer, thereby forming an internal distribution network and eliminating the need for discrete electrical connections to be made to each conductive strip in the transducer. The strips are preferably made of a flexible dielectric material having an electrically conductive layer on selected outer surfaces thereof. The dielectric strips and spacers are preferably formed from a common dielectric layer using microelectronic fabrication techniques to thereby greatly simplify fabrication and avoid the need for assembling a myriad of microscopic elements.
By integrating large numbers of electrically conductive strips and spacers into a single device, certain advantages can be realized. The force exerted by the transducer and the range of motion of the transducer can both be increased. However, these integrated devices may be subject to weaknesses caused by a weak pair of conductive strips. Nonfunctional pairs of conductive strips may stretch apart under a load thereby absorbing a part of the contraction of the functioning pairs of conductive strips within the array. Uneven load distribution may result in localized distortions which decrease the force exerted by the transducer. In addition, the resistor/capacitor time constant may result in a time delay when charging the device. Furthermore, the fine nature of the transducer elements may result in damage due to handling, attachment to larger systems and operation over an extended operational lifetime.
Further advances are described in the paper entitled Integrated Force Arrays, Bobbio et al., IEEE Micro Electro Mechanical Systems Proceedings, February, 1993, the disclosure of which is incorporated herein by reference. The transducer elements are arranged into cells and arrays of cells which are surrounded by a more substantial supporting structure. The supporting structure leads to an overall improvement in robustness, improved load distribution, and amelioration of irregularities in cell performance. The cells may be arranged into modular or fractal arrays. These arrangements make it possible to trade force for compression by connecting the cells differently. This supporting structure, however, may resist the contraction of the transducer thereby reducing the force exerted by the transducer and increasing the power consumed.